function move(id,spd){
 var obj=document.getElementById(id),max=-obj.offsetHeight+obj.parentNode.offsetHeight,top=parseInt(obj.style.top);
 if ((spd>0&&top<=0)||(spd<0&&top>=max)){
  obj.style.top=top+spd+"px";
  move.to=setTimeout(function(){ move(id,spd); },20);
 }
 else {
  obj.style.top=(spd>0?0:max)+"px";
 }
}